Fluid/solid contacting columns comprising a plurality of vertically spaced perforated plates in a column and wherein solid adsorbent is fed to the top of the column and loaded solution to the bottom of the column to effect multi-stage countercurrent extraction of the required dissolved materials, are well known.
One such type of column is described in U.S. Pat. No. 2,742,381 to Weiss et al. In this type of column, which operates on a continuous countercurrent flow with the solid absorbent being retained on the plates by weirs on the edges of plates or trays which do not span the entire cross-sectioned area of the column to leave communication passages between stages, there is little or no control of the flow of the adsorbent as the onward flow of solids down the column is dependant on the size and density of the individual solid particles. Thus lighter and smaller particles will flow through the column preferentially whilst heavy particles can become held up in a stage. With this arrangement solid adsorbent is only transferred from one stage to another by overflow i.e. when an excess is present on the tray.
Another type of column is described and claimed in U.S. Pat. No. 3,957,635 entitled "Contacting liquids and solids in countercurrent". In this type of column the plates or trays are perforated and span the entire cross-sectioned area of the column. The perforations may be fitted with bubble caps.
Columns of the latter general type, and which are of concern here, operate by causing the fluid to flow in opposite directions intermittently with velocities chosen to provide different parts of a composite cycle which include; a first part which comprises forward flow of liquid up the column during which adsorption takes place and the solid adsorbent is fluidised; a second part in which liquid flow is ceased and solid adsorbent is allowed to settle onto the plates or trays; and a third part in which flow of liquid is reversed to cause solid adsorbent to move to a lower plate or tray irrespective of the size or weight of individual particles, this third part being followed by the first part of the next cycle.
Ideally, and in order to prevent unwanted backmixing of the solid adsorbent, all of the adsorbent on one plate or tray would be moved to the next lower plate or tray during the third part of a cycle. The reason for this is that backmixing results in a loss of efficiency of the process. Also, it is important that each plate or tray has substantially the same quantity of solid adsorbent above it so that, in effect, a plug type of flow can be set up during the third part of the cycle.
Prior to the invention of U.S. Pat. No. 3,957,635, difficulties were experienced in that there tended to be a loss of solid adsorbent from the bottom stage of the column in consequence of excessive quantities of adsorbent leaving through the lowermost stage or plate. This was overcome by the expedient set forth and claimed in said U.S. patent, namely by limiting the quantity of solid adsorbent which could pass through the lowermost plate or tray to a quantity less than that passing through the other plates or trays. The forward or first part of a cycle would then result in a redistribution of the solid absorbent over the various stages. These columns operated effectively with the types of solid adsorbent, in particular ion exchange resins, employed heretofore.
However, it has now been found that, in some cases, an excessive accumulation of solid adsorbent, such as activated carbon, when the particles are of a less regular shape, can develop in the lower stages of the column.
It is the object of this invention to provide a method of operating a column of the above type as well as an improved column in which the difficulties experienced at least with certain types of solid adsorbent are overcome or, at least, diminished.